The invention relates to a radio receiver in which the received signal is converted by mixing into an intermediate frequency signal and/or a baseband signal, and in which an apparatus is provided which generates an intermediate frequency signal and/or baseband signal and derives therefrom a control signal for suppression or reduction of interfering disturbances.
As is well-known, the purpose of radio receivers is to receive electromagnetic waves. Radio receivers are, for example, radio broadcast receivers, television receivers or radiotelephones.
The problem that a signal spectrum whose spectral components may exhibit level differences of up to 120 dB must be processed by the receiver is known to occur in radio receivers. The high-level signal components mostly result in interfering disturbances such as, for example, multiple reception through harmonic mixing and intermodulation. Such disturbances are known to be caused by the signal-dependent driving of the non-linear components located in the signal path.
The intermodulation disturbances are particularly critical since they may already occur at a relatively low disturbance signal level. Intermodulation disturbances are disturbances which are caused by at least two disturbance signals and constitute a disturbance when the frequencies of, for example, two disturbance signals with the frequencies f.sub.s1 and f.sub.s2, respectively, have such a constellation in relation to each other that one of the two conditions EQU 2f.sub.s1 -f.sub.s2 =f.sub.e or 2f.sub.s2 -f.sub.s1 =f.sub.e
is met, with f.sub.e being the frequency of the desired signal or the set receiving frequency.
In this case, an "apparent" desired signal which - in the case of two disturbance signals - typically contains the modulation content of both disturbance signals, may be received on the desired frequency. Such a reception situation is then mostly interpreted by the user, for example, the radio broadcast listener, as lacking selectivity of the receiver. Another disturbing effect of the intermodulation is the interference formation with a weaker desired signal, which could be satisfactorily received, for example, without intermodulation or at low intermodulation. The danger of the disturbing effect of high-level signal components at the receiver input generally increases overproportionally with the number of high-level signal components and with their level. In most cases, intermodulation disturbances in a radio receiver are formed in the receiver stages before the channel selection, i.e., in the receiver prestage (RF) or in the mixing stage. Components which cause such disturbances are, for example, bipolar transistors, field-effect transistors and diodes; varactor diodes are also included in the intermodulation forming components.
In technical literature, the intermodulation behavior of a radio receiver is characterized by the so-called intercept point. It is the third order intercept point that is meant. The intercept point is apparent from the diagram of FIG. 1. Indicated in this diagram, on the abscissa axis are the level of the desired signal P.sub.e and also the levels P.sub.s1 and P.sub.s2 of the two disturbance signals causing the intermodulation, and on the ordinate axis, as an example, the intermediate frequency signal level P.sub.if at the output of the receiver mixing stage. Curve 1 shows the output signal level P.sub.if in dependence upon the desired signal P.sub.e. Curve 2 describes the dependency of the output level P.sub.if on disturbance signal levels P.sub.s1 and P.sub.s2, which cause the intermodulation (3rd order). In the illustration of the dependency it is assumed that both axes of FIG. 1 have logarithmic graduation, that the two disturbance signal levels are, furthermore, identical in size and that there is also no amplification control in the signal path. It is, furthermore, assumed that the level at which a signal/noise ratio of 30 dB occurs at the receiver output, in relation to a predetermined desired signal modulation, is regarded as the smallest desired signal level in the diagram (zero point of the abscissa axis). The point of intersection of the two curve tangents produces a fictitious point in the diagram, the so-called intercept point, which is associated with a certain input level, the fictitious level of the disturbance signals forming the intermodulation and a certain fictitious IF output level. The gradients of the two curve tangents typically differ by the factor 3. In radio receivers, the intercept point is generally indicated in relation to the receiver input level.
A large level value of the intercept point is aimed at for a radio receiver. The larger this value is, the larger are the disturbance signal levels which the receiver can process without a disturbing effect through intermodulation. An increase in the intercept level of a receiver is, however, limited by economic considerations.
In order to reduce intermodulation disturbances or, more generally, interfering disturbances, it is known to control the amplification at the receiver input in dependence upon the input signal, for example, by controlling an amplification component or by controlling a damping member comprised, for example, of PIN diodes. In known radio receivers, the control signal controlling the amplification or damping is produced, for example, by rectification of the amplified intermediate frequency signal and/or by rectification of the signal before the channel selection, for example, via the output of the prestage or via the input or the output of the mixing stage.
The positive effect of such a control for decreasing interfering disturbances does, however, only occur when the component controlling the amplification or the damping is arranged before the receiver stage forming the interference, and the controlled component itself does not contribute towards the interference.
A radio receiver circuit is known from U.S. Pat. No. 4,126,828, wherein a separate apparatus whose task it is to recognize intermodulation disturbance and which supplies a control signal for automatic damping of the antenna signal, is provided for reduction of intermodulation disturbances. This apparatus consists of a mixer which is fed by a non-linear circuit stage. The mixer receives its second signal from the oscillator of the receiver. The non-linear apparatus is controlled by the output of the high frequency stage of the radio receiver. The output signal of the mixer controls an intermediate frequency amplifier whose output signal is rectified. The rectified signal is compared in a comparator circuit with the rectified intermediate frequency signal of the radio receiver. If the rectified signal of the apparatus is larger than the rectified intermediate frequency signal of the radio receiver, a control signal is generated for damping the antenna signal. The damping of the antenna signal is effected to such an extent that the two rectified signals assume almost the same value. The effect produced by the larger non-linearity of the separate apparatus in comparison to the non-linearity of the radio receiving section is that the intermediate frequency signal effected by intermodulation forming disturbance signals may become larger than the intermediate frequency signal corresponding to the desired received signal.
The disadvantage of the known circuit is that in the presence of disturbance signals which do not lead to disturbing intermodulation, or in the case of strong desired signals, no signal attenuation is effected. This results in sequential disturbances such as, for example, modulation distortions or cross modulation.